This invention relates to a planar display device for displaying a monochromatic or color image as a liquid crystal display, plasma display, light-emitting diode display, etc. with a plurality of display elements arranged in rows and columns.
As the prior art, a color liquid crystal display device will be described to point out problems in this type of planar display device.
Referring to FIG. 1, there is shown a liquid crystal display device which comprises a pair of transparent substrates 11 and 12 and liquid crystal 13 sealed therebetween. A plurality of transparent square display electrodes 1.sub.l,n (l=1, 2, 3, . . . , n=1, 2, 3, . . . ) are provided on the inner surface of one of the transparent substrates, i.e., substrate 11. A transparent common electrode 14 is provided on the entire inner surface of the other substrate 12.
The display electrodes 1.sub.l,n are arranged in rows and columns. As shown in FIG. 2, a row drive line 2.sub.l is provided along a corresponding one of the rows of display electrodes 1.sub.l,n, and a column drive line 3.sub.n is provided along corresponding one of columns of display electrodes 1.sub.l,n. A thin-film transistor 4.sub.l,n is provided for each display electrode 1.sub.l,n. Each thin-film transistor 4.sub.l,n has a drain connected to the corresponding display electrode 1.sub.l,n, a gate connected to the corresponding row drive line 2.sub.l and a source connected to the corresponding column drive line 3.sub.n. Thus, when one row drive line 2.sub.l and one column drive line 3.sub.n are selectively driven, only the thin-film transistor 1.sub.l,n connected to these row and column lines is turned on, i.e., rendered conductive. The corresponding display electrode 1.sub.l,n is thus connected to the column drive line 3.sub.n, and a voltage is applied between the display electrode 1.sub.l,n and the common electrode 14 (FIG. 1). The pertaining portion of the liquid crystal 13 thus is controlled so that it has different light transmission characteristics from those of the rest of the liquid crystal. In this manner, voltage is selectively applied to the plurality of display electrodes 1.sub.l,n according to an image to be displayed, whereby a monochromatic pixel display is obtained. Each of the display electrodes 1.sub.l,n and the corresponding one of the thin-film transistors 4.sub.l,n, the corresponding portion of liquid crystal 13 and the common electrode 14 constitute, in all, one of display elements 5.sub.l,n.
For the color display, a red filter R, a green filter G and a blue filter B are provided on either respective display electrodes 1.sub.l,n or on the corresponding portions of the common electrode 14. These color filters are arranged substantially uniformly, for instance as shown in FIG. 3. Various colors can be displayed as mixtures of the red, green and blue colors depending on the state of display by the plurality of display elements corresponding to the respective display electrodes. Hereinafter, the display elements for displaying the red color will be referred to as R, the display elements for displaying the green color as G, and the display elements for displaying the blue color as B.
For displaying a white picture point (i.e., a white dot) on the planar color display device, three color display elements, i.e., red, green and blue display elements adjacent to one another, have to be driven simultaneously for white color emission. White horizontal and vertical lines can be displayed simply by activating the corresponding row and column of color display elements R, G and B. A 45-degree white oblique line from the right top to the left bottom of the display device can also be displayed by selectively activating color display elements R, G and B along the oblique line, as shown in FIG. 4. However, when color display elements are selected along a 45-degree oblique line from the left top to the right bottom on the display device, only one of the three colors, e.g. red display elements R are displayed and a white line can not be display, as shown in FIG. 5. This problem arises if it is intended to have one picture element (i.e., point, dot or pixel) constituted by one display element, i.e., if each display element is intended to be used as a resolvable picture element so that a thin oblique or curved display line can be achieved.
From this standpoint, it is desired to adopt a three-color display element set for a picture dot, in which a set of three adjacent color display elements, i.e., red, green and blue color display elements R, G and B, are simultaneously driven for display of a white picture point, and also any other desired color is displayed as a picture point (i.e., dot) of a resultant color of suitable combination of light intensities through the three color display elements. To this end, it is possible to form sets of color display elements using each two adjacent rows of color display elements as shown in FIG. 6. More specifically, it can be arranged to have adjacent red, green and blue display elements R, G and B in two adjacent element rows as a set, as shown in FIG. 6, thus defining color display element sets each shown enclosed by a phantom line, these sets constituting respective picture points P.sub.i,j (i=1, 2, 3, . . . , j=1, 2, 3, . . .)
For the display on the planar display device, one row drive line 2.sub.l is selectively driven via a row drive circuit 17 according to the contents of a row register 16, while one column drive line 3.sub.n is selectively driven via a column drive circuit 19 according to the contents of a column register 18, as shown in FIG. 2, thus causing the display of a corresponding display electrode. In the column register 18, video signal data for one display line is stored in correspondence to individual display elements 5.sub.l,n of the display line. After the display of this line, the next row drive line is selectively driven, and image signal data for the next line of the display element row to be displayed is stored in the column register 18. Likewise, successive row drive lines are selectively driven while storing image signal data for a line in the column register 18 after selection of each row drive line.
For the display through representation by sets of three-color display elements as respective picture points as shown in FIG. 6 using the system of FIG. 2, one display row 6.sub.i is displayed as follows. As the image signal, three color element signals R.sub.k, G.sub.k and B.sub.k (k=1, 2, 3, . . . ) for each picture point (i.e., dot) are supplied as parallel signals, as shown in FIG. 7. Each set of these three color element signals will be referred to as a pixel signal or dot signal, and a color video signal comprises a series of pixel signals. The individual pixel signals in the video signal for one display row are divided into two signals, i.e., one being a stream of color element signals R.sub.1, B.sub.1, G.sub.2, R.sub.3, B.sub.3, G.sub.4, . . . loaded in the column register 18 as shown in FIG. 8A and the other being a stream of color element signals G.sub.1, R.sub.2, B.sub.2, G.sub.3, R.sub.4, B.sub.4, . . . as shown in FIG. 8B. First, the signals shown in FIG. 8A stored in the column register 18 in FIG. 2 are provided to activate the color display elements connected to the corresponding row drive line 2.sub.l and individual column drive lines 3.sub.n, 3.sub.n+1, 3.sub.n+2, . . . . Then, the signals shown in FIG. 8B stored in the column register 18 are provided to activate the color display elements connected to the row drive line 2.sub.l+1. In the above way, the display signal for one display row (i.e., one horizontal scanning line cycle) is divided into two streams of color element signals for driving display elements independently. Therefore, the operation is complicated. Besides, since the video signal is usually supplied for each display row, i.e., each horizontal scanning line, the aforementioned display system is inferior in view of the matching with the divided two streams of input video signals.
Furthermore, in the planar display device the display surface is repeatedly scanned by selecting successive row drive lines. If the repetition cycle period of scanning the display area (i.e., vertical cycle period), i.e., one frame display period, is long, flicker of the display surface screen occurs to deteriorate the quality of display. For this reason, it is difficult to set the vertical cycle period to be longer than about 1/50 second. Since the vertical cycle period is fixed, by increasing the row drive lines the period of driving one row drive line is reduced. Therefore, this leads to a problem in the case of a liquid crystal display drive in that display electrodes fail to be charged sufficiently. That is, there is an upper limit on the number of row drive lines, and the resolution can not be improved beyond this limit. Even in case of a display device having high response speed compared to the liquid crystal display device, increasing the row drive lines requires an increase in the rate of switching of the two drive lines, thus leading to expensive and complicated peripheral circuits.